1. Field of the Invention
The invention relates to an optical device, more particularly to an optical multilayer film and a backlight module utilizing the same.
2. Description of the Related Art
As shown in FIG. 1, a conventional backlight module serving as a planar light source for a liquid crystal display (LCD) includes a brightness enhancement film (BEF) 11 for enhancing luminance brightness of on-axis light beams, and at least one diffuser film 12 disposed adjacent to the brightness enhancement film 11 for homogenizing light beams incident thereupon which is emitted from a light source unit 14 or reflected by a reflector 15. The on-axis light beams is referred to as light beams that travel substantially parallel to an optical axis (OA′) of the backlight module, i.e., light beams that emerge substantially perpendicularly from the backlight module.
The brightness enhancement film 11 can be a commercially available one that is manufactured by 3M United States, and that is made from a transparent material and has a smooth light-incident surface 111, and a light-exit surface 112 formed with a prismatic structured array. The prismatic structured array has the function of directing off-axis light beams (i.e., light beams traveling divergently from the optical axis (OA′)) toward the optical axis (OA′), thereby homogenizing the light beams and enhancing the brightness of illumination provided by the conventional backlight module.
Moreover, a tiny air gap 13 is often purposely left between the brightness enhancement film 11 and the diffuser film 12 in order to enhance the ability to direct off-axis light beams toward the optical axis (OA′). Difference between the refractive index of air in the air gap 13 and that of the brightness enhancement film 11 directs the off-axis light beams toward the optical axis (OA′) as the off-axis light beams travel through the air gap 13 to the light-incident surface 111 of the brightness enhancement film 11.
The air gap 13 naturally exists between the brightness enhancement film 11 and the diffuser film 12 with no additional means necessary when the two films 11, 12 are assembled into a backlight module.
However, since no structural support or coupling means is present between the brightness enhancement and diffuser films 11, 12, width of the air gap 13 (i.e., the interval between the brightness enhancement and diffuser films 11, 12) varies easily due to deformation of the brightness enhancement and diffuser films 11, 12. It is even possible for the air gap 13 to disappear if the brightness enhancement film 11 and the diffuser film 12 are in direct contact with each other, which deteriorates refraction characteristic at the light-incident surface 111 of the brightness enhancement film 11.
Moreover, the brightness enhancement film 11 and the diffuser film 12 are required to be assembled separately in different steps during assembly of the conventional backlight module. This assembly procedure is not only relatively troublesome, but it is also possible for dust to enter the air gap 13 between the brightness enhancement film 11 and the diffuser film 12 such that the overall optical characteristics of the conventional backlight module are degraded.